Colorectal cancer is a leading cause of cancer-related deaths worldwide, and is the second leading cause of cancer-related deaths in the United States. Although a patient's prognosis is good if the cancer is caught early, when the site of the cancer is confined to its site of origin, cure rates fall once the cancer has spread. Screening measures such as sigmoidoscopy and colonoscopy have demonstrated a high rate of early detection of colorectal cancer, but are expensive and can cause a great deal of patient discomfort.
A commonly used and much less expensive way of screening for colorectal cancer is a fecal occult blood test (FOBT), which tests for the presence of hemoglobin in feces. The presence of hemoglobin in feces is an indicator of intestinal bleeding, which is frequently associated with colorectal cancer. When such fecal blood is detected, a patient can be referred for further medical testing.
FOBT tests fall primarily into two categories, immunochemical tests and tests based on the use of chromogenic chemical reagents such as gum guaiac. Immunochemical tests specifically detect the presence of human hemoglobin in a fecal sample. However, they are more expensive than comparable chemical tests and require more time to obtain a result. In addition, human hemoglobin in fecal samples degrades with time, resulting in a loss of antigenicity which can produce false negative results.
Chemical FOBT tests are, on the other hand, faster and less expensive. The reagents used in such tests, however, sometimes react with animal blood or other substances in fecal samples, producing false positive results at a rate estimated to be in the range of 2%-5%. In order to rule out the possibility of a false positive test result when a chemical FOBT is used, an immunoassay can be performed (see, e.g., U.S. Pat. No. 5,747,344). However, such follow-up immunochemical testing is performed with a different sample of a patient's feces. Since blood may not be present uniformly throughout a fecal specimen or sample, a false negative result is possible, potentially resulting in a patient's cancer going undetected.